The isolation of human myelin basic protein (HBP) has been modified to improve the quality of the preparation. HBP prepared by our standard large-scale procedure is always partially degraded and all preparations are contaminated with a neutral proteinase such that incubation at 37C for 24 h causes complete degradation of the BP. BP extracted directly from human myelin has less enzyme activity but is nevertheless extensively degraded. Because of the enzymatic contaminant in the batch preparation, it is unsuitable for the further purification essential for preparation of purified BP-fragments. By modifying our current batch procedure, we have been able to prepare HBP from whole CNS with negligible amounts of enzyme activity. However, the purified HBP still contains more degradation products that other mammalian BPs prepared by the new procedure. The possible significance of these fragments in vivo is under investigation.